/Engine/Plugins/Online/OnlineFramework/Source/Lobby/Public/LobbyBeaconHost.h
Notification that a client has been disconnected from the host in some way (timeout, client initiated, etc)
virtual void NotifyClientDisconnected ( [AOnlineBeaconClient](API\Plugins\OnlineSubsystemUtils\AOnlineBeaconClient) * LeavingClientActor )